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arXiv:2012.04086 · Heliyon

Nonlocal realism tests and quantum state tomography in Sagnac-based type-II polarization-entanglement SPDC-source

Ali Motazedifard, S.A. Madani, J.J. Dashkasan, N.S. Vayaghan

Published Jun 1, 2021• 17 citations
arXivHTMLPDF

Abstract

We have experimentally created a robust, ultrabright and phase-stable polarization-entangled state close to maximally entangled Bell-state with %98-fidelity using the type-II spontaneous parametric down-conversion (SPDC) process in periodically-poled KTiOPO 4 (PPKTP) collinear crystal inside a Sagnac interferometer (SI). Bell inequality measurement, Freedman's test, as the different versions of CHSH inequality, and also visibility test which all can be seen as the nonlocal realism tests, imply that our created entangled state shows a strong violation from the classical physics or any hidden-variable theory. We have obtained very reliable and very strong Bell violation as S = 2.78 ± 0.01 with high brightness V HV = % \( 99.969 ± 0.003 \) and V DA = % \( 96.751 ± 0.002 \) and very strong violation due to Freedman test as δ F = 0.01715 ± 0.00001 . Furthermore, using the tomographic reconstruction of quantum states together a maximum-likelihood-technique (MLT) as the numerical optimization, we obtain the physical non-negative definite density operator which shows the nonseparability and entanglement of our prepared state. By having the maximum likelihood density operator, we calculate some important entanglement-measures and entanglement entropies. The Sagnac configuration provides bidirectional crystal pumping yields to high-rate entanglement source which is very applicable in quantum communication, sensing and metrology as well as quantum information protocols, and has potential to be used in quantum illumination-based LIDAR and free-space quantum key distribution (QKD).

DOI
10.1016/j.heliyon.2021.e07384

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•Jan 1, 2026•0 citations
We have experimentally created a robust, ultrabright and phase-stable polarization-entangled state close to maximally entangled Bell-state with %98-fidelity using the type-II spontaneous parametric down-conversion (SPDC) process in periodically-poled KTiOPO 4 (PPKTP) collinear crystal inside a Sagnac interferometer (SI). Bell inequality measurement, Freedman's test, as the different versions of CHSH inequality, and also visibility test which all can be seen as the nonlocal realism tests, imply that our created entangled state shows a strong violation from the classical physics or any hidden-variable theory. We have obtained very reliable and very strong Bell violation as S = 2.78 ± 0.01 with high brightness V HV = % ( 99.969 ± 0.003 ) and V DA = % ( 96.751 ± 0.002 ) and very strong violation due to Freedman test as δ F = 0.01715 ± 0.00001 . Furthermore, using the tomographic reconstruction of quantum states together a maximum-likelihood-technique (MLT) as the numerical optimization, we obtain the physical non-negative definite density operator which shows the nonseparability and entanglement of our prepared state. By having the maximum likelihood density operator, we calculate some important entanglement-measures and entanglement entropies. The Sagnac configuration provides bidirectional crystal pumping yields to high-rate entanglement source which is very applicable in quantum communication, sensing and metrology as well as quantum information protocols, and has potential to be used in quantum illumination-based LIDAR and free-space quantum key distribution (QKD).
Entangled photons are crucial for the development of quantum technologies and especially important in quantum communications. Hence it is paramount to have a reliable, high-fidelity source of entangled photons. Here we describe the construction and characterization of a polarization-entangled photon source. We generate maximally-entangled Bell states using a type-II PPKTP nonlinear crystal inside a Sagnac interferometer. We characterize the source in terms of brightness, visibility, Bell-CHSH test and we perform quantum-state tomography of the density matrix. Our source violates Bell-CHSH inequality ∣S∣≤2|S|\le 2∣S∣≤2 by n=22n=22n=22 standard deviations. With visibilities up to V=98.9%{\cal V}= 98.9\%V=98.9% and fidelity F=97%{\cal F}= 97\%F=97%, our source is highly competitive, with state-of-the-art performance. To the best of our knowledge, this is the first source of entangled-photons designed and build in Romania and as such, represents an important step for the development of quantum technologies in our country. We envisage that our results will stimulate the progress of quantum technologies in Romania and will educate the future generation of quantum engineers.
Abstract Using the type-I SPDC process in BBO nonlinear crystal (NLC), we generate a polarization-entangled state near to the maximally-entangled Bell-state with high-visibility (high-brightness) 98.50 ±1.33 % (87.71 ± 4.45 %) for HV (DA) basis. We calculate the CHSH version of the Bell inequality, as a nonlocal realism test, and find a strong violation from the classical physics or any hidden variable theory (HVT), S= 2.71±0.10. Via measuring the coincidence count (CC) rate in the SPDC process, we obtain the quantum efficiency of single-photon detectors (SPDs) around (25.5 ±3.4)% , which is in good agreement to their manufacturer company. As expected, we verify the linear dependency of the CC rate vs. pump power of input CW-laser, which may yield to find the effective second-order susceptibility crystal. Using the theory of the measurement of qubits, includes a tomographic reconstruction of quantum states due to the linear set of 16 polarization-measurement, together with a maximum-likelihood-technique (MLT), which is based on the numerical optimization, we calculate the physical non-negative definite density matrices, which implies on the non-separability and entanglement of prepared state. By having the maximum likelihood density operator, we calculate precisely the entanglement measures such as Concurrence, entanglement of formation, tangle, logarithmic negativity, and different entanglement entropies such as linear entropy, Von-Neumann entropy, and Renyi 2-entropy. Finally, this high-brightness and low-rate entangled photons source can be used for short-range quantum measurements in the Lab.
Entangled photon source is a basic building block of various quantum technologies such as quantum communication, QKD, quantum metrology, etc. Spontaneous parametric down conversion (SPDC) process is widely reported method for generation of entanglement due to its ease of implementation and stable performance. In this paper, we report design and development of type-II Sagnac loop based entangled photon source with a high brightness of 8.42×1048.42\times 10^{4}8.42×104 pairs/s/mW with 98.18% visibility, a CHSH inequality violation of S=2.777±0.006 and state fidelity of 97.8%. An automated test setup has also been developed for carrying out the measurements. The essential formulated design procedure to achieve optimal performance has been discussed and presented. The measured performance of developed source is also compared to other reported sources having the same configuration. In this work, higher brightness is achieved with comparable fidelity to previously reported works. This methodology can be extended to any collinear Sagnac loop based entangled photon sources.
We demonstrate a polarization-entangled photon-pair source obtained via a type-II non-collinear quasi-phase-matched spontaneous parametric down-conversion process with a 10-mm periodically poled KTiOPO4 crystal, which is as stable and wavelength-tunable as the well-known Sagnac configuration scheme. A brightness of 4.2 kHz/mW is detected and a concurrence of 0.975 is estimated using quantum state tomography. Without loss of entanglement and brightness, the photon-pair wavelengths are tunable through control of the crystal temperature. This improvement is achieved using the non-collinear configuration and a stable interferometric distinguishability compensator.
We report a source of polarization-entangled photon pairs in the 1550-nm telecommunication band, which is based on non-collinear spontaneous parametric down-conversion in a periodically poled lithium niobate crystal pumped by picosecond pulses. This source is realized utilizing a polarization-based Sagnac interferometer employing a type-0 non-collinear quasi-phase-matching configuration. Polarization entanglement is verified through measurement of the polarization-correlation interference fringes with visibility >96% and by testing the experimental violation of the Clauser-Horne-Shimony-Holt (CHSH) form of Bell's inequality. The CHSH-Bell parameter S is found to be 2.72 ± 0.04, with 18 standard deviations from the statistical uncertainty.
This thesis presents the characterization of SagnaQ, a fiber-based polarization-entangled photon pair source developed at Quantum Scopes AB. The source generates photon pairs at telecom wavelengths through type-0 Spontaneous Parametric Down-Conversion (SPDC) in a Periodically Poled Lithium Niobate (PPLN) waveguide crystal placed inside a Sagnac interferometer loop. The purely fiber-coupled design moves towards a compact, plug-and-play platform, offering greater stability and ease of use compared to free-space implementations.The generated quantum state was characterized through Quantum State Tomography (QST), polarization correlation measurements, a Clauser-Horne-Shimony-Holt (CHSH) inequality test, and heralded second-order correlation function measurements. The reconstructed density matrix yields a fidelity of 85 %85\,\%85% to the ∣Φ−⟩|\Phi^{-}\rangle∣Φ−⟩ Bell state, with a purity of 0.759±0.0080.759 \pm 0.0080.759±0.008 and a concurrence of 0.715±0.0080.715 \pm 0.0080.715±0.008. Polarization correlation visibilities of 0.929±0.0180.929 \pm 0.0180.929±0.018 and 0.790±0.0110.790 \pm 0.0110.790±0.011 were measured in the HV and DA bases respectively, and the CHSH parameter was found to be S=2.3301±0.0075S = 2.3301 \pm 0.0075S=2.3301±0.0075, violating the classical bound by more than 44 standard deviations. A heralded second-order correlation function of gH(2)(0)=0.087±0.002g_{H}^{(2)}(0) = 0.087 \pm 0.002gH(2)​(0)=0.087±0.002 confirms the single-photon character of the source. Power-dependent measurements demonstrate a linear scaling of coincidence count rates with pump power, with a degradation of state quality observed at higher pump powers.